As a mobile communications system, for example, a cellular system is known. A cellular system is a system to secure a communication range of a service area for a wide range by laying out cells (communication area of base stations which cover from several hundreds meters to several kilometers). By the way, in case of a cellular system, in order to avoid the situation where communication is impossible due to dark place or an increase of number of terminals in a service area, countermeasures which increase the number of the base stations are being performed. In that case, there are variously arrangement patterns of the base stations (for example, a case where a cover area of a certain cell overlaps with a cover area of a cell which is adjacent to the certain cell, a case where entire cover area of the certain cell is included in a cover area of other cell, and a case where each cover area of three or more cells overlaps).
However, the overlap and the inclusion of the cover area might cause the generation of radio wave interference between the base stations. Radio interference brings about lowering of channel capacity. Also, with an increase of number of base stations, in spite of a mobile station not existing in a cover area of the cell (that is, in spite of not being used at all), probability rises that a base station which is kept started exists. Electric power is wasted by a base station which is being started wastefully and without being used at all.
Accordingly, technologies aiming at interference avoidance or power savings in a mobile communications system are proposed. For example, Japanese Patent Application. Laid-Open No. 2003-37555 describes a technology in which a certain radio base station monitors a transmission signal transmitted from other radio base stations, and by considering traffic status and received power of other radio base stations, suspends transmission or starts transmission of own base station. The publication mentions that the number of base stations operating in low traffic and the interference in surrounding base stations are decreased. Further, the publication discloses a base station which, when traffic of other base stations is heavy, makes own base station return from a sleep state to a normal state, and covers at least one mobile station which cannot be accommodated by other stations in heavy traffic.